A general projection display apparatus includes at least a light source, an illumination optical system, an image-forming device, and a projection optical system. The illumination optical system introduces light emitted from the light source to the image-forming device and illuminates the image-forming device. The image-forming device forms an image (an optical image) by modulating illumination light based on an image signal. The projection optical system projects the image formed by the image-forming device.
When a black image is displayed (i.e., projected) by the project display apparatus that uses a liquid crystal panel as the image-forming device, relatively bright and dark areas appears (uneven brightness appears) on the displayed image. This is because of the following reasons: Typically, polarizing plates are arranged on the incidence side and the exit side of the liquid crystal panel, respectively, so that the transmission axes of the polarizing plates intersect each other at right angles. Light entering the liquid crystal panel is preliminarily converted into linear polarized light. When a white image is displayed, the polarization axis of light entering the liquid crystal panel through the incidence-size polarizing plate rotates 90 degrees. Then, the polarization axis of light exiting the liquid crystal panel is parallel to the transmission axis of the exit-side polarizing plate, so that light exiting the liquid crystal panel passes through the exit-side polarizing plate. The polarization axis of light entering the liquid crystal panel cannot be rotated when the black image is displayed. Then, the polarization axis of light exiting the liquid crystal panel and the transmission axis of the exit-side polarizing plate are at right angle to each other, so that light exiting the liquid crystal panel is interrupted (absorbed) by the exit-side polarizing plate.
However, in practice, part of light exiting the liquid crystal panel is converted into elliptical polarized light due to pre-tilt angles of the incidence side and the exit side on the liquid crystal panel (a liquid crystal layer). More specifically, part of light fluxes exiting the liquid crystal panel is converted into elliptical polarized light. Due to optical anisotropy of the liquid crystal panel, a phase difference occurs in part of the exiting light fluxes. As a result, part of the light fluxes converted into elliptically-polarized light passes through the exit-side polarizing plate, and thus uneven brightness takes place.
When uneven brightness occurs in the black image as described above, the brightness of the entire image rises compared to the case where uneven brightness is not occurring. This, as a result, decreases the difference in brightness between the black image and the white image. In other words, contrast decreases.
In order to solve the foregoing problems, in some types of projection display apparatuses, an optical compensation plate having optical anisotropy opposite to optical anisotropy of a liquate crystal panel is interposed between an incident-side polarizing plate and an exit-side polarizing plate. In detail, the optical anisotropy of the liquid crystal panel is a phase difference generally caused by the pre-tilt angle of a liquid crystal layer. To compensate for the phase difference, the optical compensation plate is interposed. In other words, elliptical polarized light which occurred due to light passing through a liquid crystal panel is converted into liner polarized light. Particulars of the optical compensation plate are disclosed in Japanese laid-open publication No. H08-50206.
In the projection display apparatuses having higher luminance, the amount of light emitted onto the exit-side polarizing plate of the liquid crystal panel increases. When a greater amount of light is emitted onto the polarizing plate, a greater amount of light is absorbed by the polarizing plate. The greater amount of light that is absorbed also increases the amount of heat generation. This, as a result, accelerates deterioration of the exit-side polarizing plate thereby decreasing lifetime.
Accordingly, Japanese laid-open publication No. 2002-72162 discloses an apparatus in which two polarizing plates are arranged on the exit side of a liquid crystal panel in order to decelerate the deterioration of respective polarizing plates by dispersing the amount of light absorption.